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Prosecutors said the boy, whose name has not been made public because of his age, was arraigned on delinquency charges of involuntary manslaughter and illegal possession of a firearm in Dorchester Juvenile Court, where proceedings are closed.

The boy’s lawyer, Michael Doolin, says his client is “very scared and very heartbroken.”

Police identified the 9-year-old Monday as Jan Marcos Pena.

Police were called to the boys’ home in the city’s Mattapan neighborhood at about 11:30 a.m. Friday and found the wounded boy. He was pronounced dead at a hospital.

The shooting is thought to be accidental.

Police are investigating how the boy got the gun and why the children weren’t in school. Their mother wasn’t home at the time.

State Secretary of Human Services John Polanowicz said Monday a social worker from the Department of Children and Families had been involved with the family and last visited the home on Jan. 30, about a week before the shooting.

“This is one of those areas where the department did all the right things in this and we just had an incredibly tragic outcome with guns that we hope to get off the street,” said Polanowicz.

Boston Mayor Martin Walsh said police would launch a gun buyback program as part of an effort to reduce the number of guns in the city.